This invention is in the field of electric power generation, and more particularly, relates to induction generator systems.
Virtually all electric power generators in current use are synchronous machines. Such generators are typically connected together to form an electric power grid. In other cases synchronous generators are operated as autonomous electric power generators. While such synchronous machines do effectively perform in the required electrical power generating applications, those machines are relatively high cost compared with other known generators, such as induction machines adapted for operation in a power generation mode.
However, in spite of the relatively low cost of induction machines, the prior art autonomous induction generator systems have been relatively costly due to the necessary electronics or magnetics required to establish a regulated voltage and frequency. Unlike a synchronous generator, an induction generator operated at fixed voltage and frequency does not allow its real current and reactive currents to independently vary. At fixed voltage and frequency, the real current from an induction generator can vary from zero to maximum with the variation of the slip frequency (i.e. difference between the electrical frequency and mechanical frequency). The reactive current required at fixed voltage and frequency remains lagging and of significant magnitude throughout the generator power range, becoming maximum at the maximum output power. Consequently, an external source of leading reactive current is required to establish an output voltage in an autonomous induction generator. This reactive source must be controllable or variable if the output voltage is to be regulated below saturation of the generator.
The principle disadvantage of the prior art autonomous induction generators has been the high cost of the power electronics and associated magnetics which are required to perform the necessary regulation. In addition, the quality of the output waveform of prior art autonomous induction generator systems has required relatively expensive power filters to meet desired spectral requirements.
The prior art grid-connected induction generators have been infrequently used because of low power factor and current surges during start up. Where an induction generator is to be connected to a power grid, the power grid fixes the induction generator voltage and frequency and acts as a sink for real power and as a source for reactive power. During generator operation, the induction generator shaft is rotated slightly faster than synchronous speed by a mechanical engine, or other primer mover. The resulting negative slip of the induction machine imposes a torque load on the mechanical engine and causes real electrical power to be generated and delivered to the grid. In such induction generators, the reactive current required to maintain the flux of the induction generator is supplied by the grid, resulting in a less than optimum power factor.
U.S. Pat. No. 3,829,758 (Studtmann) illustrates one form of induction generator which uses a voltage mode inverter for exciting an induction generator. A second known form is disclosed by Abbondanti and Brennen in "Static Exciters for Induction Generators", IEEE IAS Transactions, Vol. LA-13, No. 5, September/October 1977. In both of these prior art approaches, a large fixed capacitor is utilized across the output power lines to provide leading reactive current. According to the Studtmann patent, in this form, forced commuted SCR switches reconnect the capacitor from phase to phase such that a nominally constant D.C. voltage appears across the capacitor. In contrast, the Abbondanti and Brennen paper teaches the control of the reactive current by using fixed capacitors on each phase in combination with large controllable or nonlinear inductors which "bleed" or "steal away" the excessive leading reactive current which is not required by the induction machine or load. A switched inductor network is used in conjunction with a network for modulating the length of times which the various inductors are in the circuit. This approach minimizes the number of switches, but the cost of reactances is relatively high.
In alternative prior art configurations, U.S. Pat. Nos. 3,043,115 (Harter) 2,871,439 (Shaw) and 2,881,376 (Shaw) disclose a switched capacitor control for induction machines. However, those systems do not perform voltage regulation but rather permit the induction machine to saturate. There is no voltage regulation which was independent of the machine speed.
It is also known in the prior art to use either a binary capacitor array or an arithmetic capacitor array for controlling the reactive current in an induction generator. Binary capacitor arrays use a switchable sequence of capacitors having binary weighted values (e.g. 1C, 2C, 4C, 8C--) and arithmetic array uses switchable capacitors having the same values (e.g. 1C, 1C, 1C--). With either of these two systems, any integer value of capacitance may be attained by selectively switching in the appropriate ones of the capacitors to reach the desired value. However, for the arithmetic array, a relatively large number of capacitors is required to attain a wide range of capacitance values. In the binary array, a smaller number of capacitors is required, but the exponential nature of the required values for the capacitors requires relatively large capacitances to be used, contributing to system error due to the tolerance values associated with known forms of power capacitors.
Accordingly, it is an object of this invention to provide an improved induction generating system with a controlled reactance network.
Another object is to provide an improved induction generating system that is selectively adaptable for grid-connected or autonomous operation.
Yet another object is to provide an improved induction generating system that is selectively adaptable for grid-connected operation while providing a substantially uity power factor under unbalanced line-to-line or line-to-neutral loads.
Still another object is to provide a power network including two or more parallel connected induction generators.
Another object is to provide an improved power factor correction system with a controlled reactance network.